In establishing the "cost" of a given article, product or item of merchandise to the vendor, the invoice cost of any article, product or item of merchandise purchased at a forced, bankrupt, close-out sale, or other sale outside of the ordinary channels of trade may not be used as a basis for justifying a price lower than one based upon the replacement cost as of the date of said sale of said article, product or item of merchandise replaced through the ordinary channels of trade, unless said article, product or merchandise is kept separate from goods purchased in the ordinary channels of trade and unless said article, product or item of merchandise is advertised and sold as merchandise purchased at a forced, bankrupt, close-out sale, or by means other than through the ordinary channels of trade, and said advertising shall state the conditions under which said goods were so purchased, and the quantity of such merchandise to be sold or offered for sale.